During the morphogenesis of several animal and bacterial viruses a nucleic acid-free virus capsid (procapsid) is assembled and subsequently packages the viral nucleic acid. Our long-range objective is to understand the mechanism of virus procapsid assembly at the molecular level. To reach this goal we will use bacteriophage T7 as a model system and will pursue the following short-range objectives: (a) Isolation from bacteriophage T7-infected E. coli of precursors of the T7 procapsid. To determine the order of occurrence of procapsid precursors, we will determine their kinetics of appearance and will determine which precursors are assembled by amber mutants in a non-permissive host. (b) Characterization of procapsid precursors. To determine the mechanism by which precursors progress along the capsid assembly pathway we will characterize the precursors. Techniques for characterization include: Buoyant density and velocity ultracentrifugation, electron microscopy, partitioning in aqueous polymer multiphase systems, electrophoresis under non-denaturing conditions and sodium dodecyl sulfate polyacryamide gel electrophoresis. (c) Development of improved techniques for isolating viruses and virus precursors. These improved techniqques will be used in (a) and (b). Emphasis will be placed on new electrophoretic, ultracentrifugal and partitioning techniques for purifying precursors and new techniques for preparing precursors for electron microscopy. (d) Development of dynamic models of the capid assembly process. (e) Re-entry of procapsid precursors into the capsid assembly pathway in cell-free mixtures ("in vitro") under controlled conditions. These "in vitro" experiments will be used to test hypotheses developed in (a)-(b). "In vitro" experiments will also be used to isolated capsid precursors that my not have been detected in (a).